Controllable CSI-RS density

ABSTRACT

Methods and apparatus for configuring, in a network node of a wireless communication network, a reference signal resource used to perform channel-state information, CSI, measurements with one or more wireless devices. In an example method, a reference signal resource is aggregated in one or more of a frequency and a time domain, and a density characteristic of the aggregated reference signal resource that is to be transmitted to the one or more wireless devices is adjusted.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/345,411, filed Apr. 26, 2019, which is a 371 of InternationalApplication No. PCT/IB2017/057739, filed Dec. 7, 2017, which claims thebenefit of U.S. Provisional Application No. 62/431,743, filed Dec. 8,2016, the disclosure of which is fully incorporated herein by reference.

TECHNICAL FIELD

The disclosed subject matter relates generally to telecommunications andmore particularly to control of Channel State Information ReferenceSignal (CSI-RS) density in channels of a next generation mobile wirelesscommunication system.

BACKGROUND

The next generation mobile wireless communication system (5G or NR),will support a diverse set of use cases and a diverse set of deploymentscenarios. The latter includes deployment at both low frequencies (100sof MHz), similar to LTE today, and very high frequencies (mm waves inthe tens of GHz). At high frequencies, propagation characteristics makeachieving good coverage challenging. One solution to the coverage issueis to employ high-gain beamforming, typically in an analog manner, inorder to achieve satisfactory link budget. Beamforming will also be usedat lower frequencies (typically digital beamforming), and is expected tobe similar in nature to the already standardized 3GPP LTE system (4G).

For background purposes, some of the key aspects of LTE are described inthis section. Of particular relevance is the sub-section describingchannel state information reference signals (CSI-RS). A similar signalwill be designed also for NR, and is the subject of the presentapplication.

Note that terminology used here such as eNodeB and UE should beconsidering non-limiting and does in particular not imply a certainhierarchical relation between the two; in general, “eNodeB” could beconsidered as device 1 and “UE” device 2, and these two devicescommunicate with each other over some radio channel. Herein, we alsofocus on wireless transmissions in the downlink, but the invention isequally applicable in the uplink.

LTE and NR use OFDM in the downlink and DFT-spread OFDM or OFDM in theuplink. The basic LTE or NR downlink physical resource can thus be seenas a time-frequency grid as illustrated in FIG. 6, where each resourceelement corresponds to one OFDM subcarrier during one OFDM symbolinterval.

Moreover, as shown in FIG. 7, in the time domain, LTE downlinktransmissions are organized into radio frames of 10 milliseconds, eachradio frame consisting of ten equally-sized subframes of lengthTsubframe=1 millisecond.

Furthermore, the resource allocation in LTE is typically described interms of resource blocks, where a resource block corresponds to one slot(0.5 millisecond) in the time domain and 12 contiguous subcarriers inthe frequency domain. Resource blocks are numbered in the frequencydomain, starting with 0 from one end of the system bandwidth. For NR, aresource block is also 12 subcarriers in frequency, but the number ofOFDM symbols in the NR resource block has not yet been determined. Itwill be appreciated that the term “resource block,” as used herein, willthus refer to a block of resources spanning a certain number ofsubcarriers and a certain number of OFDM symbols—the term as used hereinmay, in some instances, refer to a different sized block of resourcesfrom what is ultimately labeled a “resource block” in the standards forNR or in the standards for some other system.

Downlink transmissions are dynamically scheduled, i.e., in each subframethe base station transmits control information about to which terminalsdata is transmitted and upon which resource blocks the data istransmitted, in the current downlink subframe. This control signaling istypically transmitted in the first 1, 2, 3 or 4 OFDM symbols in eachsubframe in LTE, and in 1 or 2 OFDM symbols in NR. A downlink systemwith 3 OFDM symbols as control is illustrated in the downlink subframeillustrated in FIG. 8.

Codebook-Based Precoding

Multi-antenna techniques can significantly increase the data rates andreliability of a wireless communication system. The performance isparticularly improved if both the transmitter and the receiver areequipped with multiple antennas, which results in a multiple-inputmultiple-output (MIMO) communication channel. Such systems and/orrelated techniques are commonly referred to as MIMO.

NR is currently evolving with MIMO support. A core component in NR isthe support of MIMO antenna deployments and MIMO related techniquesincluding beamforming at higher carrier frequencies. Currently, LTE andNR support an 8-layer spatial multiplexing mode for up to 32 Tx antennaswith channel-dependent precoding. The spatial multiplexing mode is aimedfor high data rates in favorable channel conditions. An illustration ofthe spatial multiplexing operation is provided in FIG. 9.

As seen, the information carrying symbol vector s is multiplied by anNT×r precoder matrix W, which serves to distribute the transmit energyin a subspace of the NT—(corresponding to NT antenna ports) dimensionalvector space. The precoder matrix is typically selected from a codebookof possible precoder matrices, and typically indicated by means of aprecoder matrix indicator (PMI), which specifies a unique precodermatrix in the codebook for a given number of symbol streams. The rsymbols in s each correspond to a layer and r is referred to as thetransmission rank. In this way, spatial multiplexing is achieved, sincemultiple symbols can be transmitted simultaneously over the sametime/frequency resource element (TFRE). The number of symbols r istypically adapted to suit the current channel properties.

LTE and NR use OFDM in the downlink and hence the received NR×1 vectory_(n) for a certain TFRE on subcarrier n (or alternatively data TFREnumber n) is thus modeled byy _(n) =H _(n) Ws _(n) +e _(n)

where e_(n) is a noise/interference vector obtained as realizations of arandom process. The precoder, implemented by precoder matrix W, can be awideband precoder that is constant over frequency or that is frequencyselective.

The precoder matrix is often chosen to match the characteristics of theNR×NT MIMO channel matrix H_(n), resulting in so-calledchannel-dependent precoding. This is also commonly referred to asclosed-loop precoding and essentially strives for focusing the transmitenergy into a subspace which is strong in the sense of conveying much ofthe transmitted energy to the UE. In addition, the precoder matrix mayalso be selected to strive for orthogonalizing the channel, meaning thatafter proper linear equalization at the UE, the inter-layer interferenceis reduced.

The transmission rank, and thus the number of spatially multiplexedlayers, is reflected in the number of columns of the precoder. Forefficient performance, it is important that a transmission rank thatmatches the channel properties is selected.

Channel State Information Reference Symbols (CSI-RS)

In LTE and NR, a reference symbol sequence was introduced for thepurpose of estimating channel-state information, the CSI-RS. The CSI-RSprovides several advantages over basing the CSI feedback on the commonreference symbols (CRS) which were used, for that purpose, in previousreleases. Firstly, the CSI-RS is not used for demodulation of the datasignal, and thus does not require the same density (i.e., the overheadof the CSI-RS is substantially less). Secondly, CSI-RS provides a muchmore flexible means to configure CSI feedback measurements (e.g., whichCSI-RS resource to measure on can be configured in a UE specificmanner).

By measuring on a CSI-RS, a UE can estimate the effective channel theCSI-RS is traversing, including the radio propagation channel andantenna gains. In more mathematical rigor, this implies that if a knownCSI-RS signal x is transmitted, a UE can estimate the coupling betweenthe transmitted signal and the received signal (i.e., the effectivechannel). Hence if no virtualization is performed in the transmission,the received signal y can be expressed asy=Hx+e

and the UE can estimate the effective channel H.

Up to 32 CSI-RS ports can be configured for a LTE or NR UE, that is, theUE can thus estimate the channel from up to eight transmit antennas.

An antenna port is equivalent to a reference signal resource that the UEshall use to measure the channel. Hence, a base station with twoantennas could define two CSI-RS ports, where each port is a set ofresource elements in the time frequency grid within a subframe or slot.The base station transmits each of these two reference signals from eachof the two antennas so that the UE can measure the two radio channelsand report channel state information back to the base station based onthese measurements. In LTE, CSI-RS resources with 1, 2, 4, 8, 12, 16,20, 24, 28 and 32 ports are supported.

The CSI-RS utilizes an orthogonal cover code (OCC) of length two, tooverlay two antenna ports on two consecutive REs. As seen in FIG. 10,which depicts resource element grids over an RB pair with potentialpositions for LTE Rel-9/10 UE specific RS (yellow), CSI-RS (marked witha number corresponding to the CSI-RS antenna port), and CRS (blue anddark blue), many different CSI-RS patterns are available. For the caseof 2 CSI-RS antenna ports there are 20 different patterns within asubframe. The corresponding number of patterns is 10 and 5 for 4 and 8CSI-RS antenna ports, respectively. For TDD, some additional CSI-RSpatterns are available.

The CSI reference signal configurations are given by the table below,taken from LTE specifications TS 36.211 v.12.5.0. For example, the CSIRS configuration 5 for 4 antennas ports use (k′,l′)=(9,5) in slot 1 (thesecond slot of the subframe), and according to the formulas below, port15,16, use OCC over the resource elements (k,l)=(9,5), (9,6) and port17,18 use OCC over resource elements (3,5)(3,6) respectively (assumingPRB index m=0), where k is the subcarrier index and I is the OFDM symbolindex.

The orthogonal cover code (OCC) is introduced below by the factor w_(l″)

$k = {k^{\prime} + {12m} + \left\{ {{\begin{matrix}{- 0} & {{{{for}\ p} \in \left\{ {{15},{16}} \right\}},{{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{{- 6}\ } & {{{{for}\ p} \in \left\{ {{17},{18}} \right\}},{{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{{- 1}\ } & {{{{for}\ p} \in \left\{ {{19},{20}} \right\}},{{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{{- 7}\ } & {{{{for}\ p} \in \left\{ {{21},{22}} \right\}},{{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{{- 0}\ } & {{{{for}\ p} \in \left\{ {{15},{16}} \right\}},{{extended}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{{- 3}\ } & {{{{for}\ p} \in \left\{ {{17},{18}} \right\}},{{extended}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{{- 6}\ } & {{{{for}\ p} \in \left\{ {{19},{20}} \right\}},{{extended}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{{- 9}\ } & {{{{for}\ p} \in \left\{ {{21},{22}} \right\}},{{extended}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}}\end{matrix}l} = {l^{\prime} + \left\{ {{\begin{matrix}l^{''} & \begin{matrix}{{{{CSI}\mspace{14mu}{reference}\mspace{14mu}{signal}\mspace{14mu}{configurations}\mspace{14mu} 0} - 19},} \\{{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}\end{matrix} \\{2l^{''}} & \begin{matrix}{{{{CSI}\mspace{14mu}{reference}\mspace{14mu}{signal}\mspace{14mu}{configurations}\mspace{14mu} 20} - 31},} \\{{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}\end{matrix} \\l^{''} & \begin{matrix}{{{{CSI}\mspace{14mu}{reference}\mspace{14mu}{signal}\mspace{14mu}{configurations}\mspace{14mu} 0} - 27},} \\{{extended}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}\end{matrix}\end{matrix}w_{l^{''}}} = \left\{ {{{\begin{matrix}1 & {p \in \left\{ {{15},{17},{19},{21}} \right\}} \\\left( {- 1} \right)^{l^{''}} & {p \in \left\{ {{16},\ {18},{20},{22}} \right\}}\end{matrix}l^{''}} = 0},{{1m} = 0},1,\ldots\mspace{14mu},{{N_{RB}^{DL} - {1m^{\prime}}} = {m + \left\lfloor \frac{N_{RB}^{\max,{DL}} - N_{RB}^{DL}}{2} \right\rfloor}}} \right.} \right.}} \right.}$

TABLE 6.10.5.2-1 Mapping from CSI reference signal configuration to (k′,l′) for normal cyclic prefix Number of CSI reference signals configuredCSI reference signal 1 or 2 4 8 configuration (k′, l′) n_(s) mod 2 (k′,l′) n_(s) mod 2 (k′, l′) n_(s) mod 2 Fram 0 (9, 5) 0 (9, 5) 0 (9, 5) 0 1(11, 2)  1 (11, 2)  1 (11, 2)  1 2 (9, 2) 1 (9, 2) 1 (9, 2) 1 3 (7, 2) 1(7, 2) 1 (7, 2) 1 4 (9, 5) 1 (9, 5) 1 (9, 5) 1 5 (8, 5) 0 (8, 5) 0 6(10, 2)  1 (10, 2)  1 7 (8, 2) 1 (8, 2) 1 8 (6, 2) 1 (6, 2) 1 9 (8, 5) 1(8, 5) 1 10 (3, 5) 0 11 (2, 5) 0 12 (5, 2) 1 13 (4, 2) 1 14 (3, 2) 1 15(2, 2) 1 16 (1, 2) 1 17 (0, 2) 1 18 (3, 5) 1 19 (2, 5) 1 Frame structure20 (11, 1)  1 (11, 1)  1 (11, 1)  1 type 2 only 21 (9, 1) 1 (9, 1) 1(9, 1) 1 22 (7, 1) 1 (7, 1) 1 (7, 1) 1 23 (10, 1)  1 (10, 1)  1 24(8, 1) 1 (8, 1) 1 25 (6, 1) 1 (6, 1) 1 26 (5, 1) 1 27 (4, 1) 1 28 (3, 1)1 29 (2, 1) 1 30 (1, 1) 1 31 (0, 1) 12D Antenna Arrays

In LTE, support for two-dimensional antenna arrays was introduced whereeach antenna element has an independent phase and amplitude control,thereby enabling beamforming in both in the vertical and the horizontaldimensions. Such antenna arrays may be (partly) described by the numberof antenna columns corresponding to the horizontal dimension N_(h), thenumber of antenna rows corresponding to the vertical dimension N_(v),and the number of dimensions corresponding to different polarizationsN_(p). The total number of antennas is thus N=N_(h)N_(v)N_(p). Anexample of an antenna where N_(h)=8 and N_(v)=4 is illustrated in FIG.11, which illustrates on the left side thereof a two-dimensional antennaarray of cross-polarized antenna elements (N_(P)=2), with N_(h)=4horizontal antenna elements and N_(v)=8 vertical antenna elements, andon the right side of FIG. 11 the actual port layout with 2 verticalports and 4 horizontal ports is illustrated. This could for instance beobtained by virtualizing each port by 4 vertical antenna elements.Hence, assuming cross-polarized ports are present, the UE will measure16 antenna ports in this example.

However, from a standardization perspective, the actual number ofelements antenna array is not visible to the UE, but rather the antennaports, where each port corresponds to a CSI reference signal. The UE canthus measure the channel from each of these ports. Therefore, weintroduce a 2D port layout, described by the number of antenna ports inthe horizontal dimension M_(h), the number of antenna rows correspondingto the vertical dimension M_(v) and the number of dimensionscorresponding to different polarizations M_(p). The total number ofantenna ports is thus M=M_(h)M_(v)M_(p). The mapping of these ports onto the N antenna elements is an eNB implementation issue and thus notvisible by the UE. The UE does not even know the value of N; it onlyknows the value of the number of ports M.

Precoding may be interpreted as multiplying the signal with differentbeamforming weights for each antenna port prior to transmission. Atypical approach is to tailor the precoder to the antenna form factor,i.e. taking into account M_(h), M_(v) and M_(p) when designing theprecoder codebook.

A common approach when designing precoder codebooks tailored for 2Dantenna arrays is to combine precoders tailored for a horizontal arrayand a vertical array of antenna ports respectively by means of aKronecker product. This means that (at least part of) the precoder canbe described as a function ofW _(H) ⊗⊕W _(V)

where W_(H) is a horizontal precoder taken from a (sub)-codebook X_(H)containing N_(H) codewords and similarly W_(V) is a vertical precodertaken from a (sub)-codebook X_(V) containing N_(V) codewords. The jointcodebook, denoted by X_(H)⊗X_(V), thus contains N_(H)·N_(V) codewords.The codewords of X_(H) are indexed with k=0, . . . , N_(H)−1, thecodewords of X_(V) are indexed with l=0, . . . , N_(V)−1 and thecodewords of the joint codebook X_(H)⊗X_(v) are indexed with m=N_(V)·k+lmeaning that m=0, . . . , N_(H)·N_(V)−1.

For LTE Rel-12 UE and earlier, only a codebook feedback for a 1D portlayout is supported, with 2,4 or 8 antenna ports. Hence, the codebook isdesigned assuming these ports are arranged on a straight line.

Periodic CSI Reporting on a Subset of 2D Antenna Ports

A method has been proposed to use measurements on fewer CSI-RS ports forperiodic CSI reports than measurements for the aperiodic CSI reports.

In one scenario, the periodic CSI report framework is identical tolegacy terminal periodic CSI report framework. Hence, periodic CSIreports with 2,4 or 8 CSI-RS ports are used for the P-CSI reporting andadditional ports are used for the A-CSI reporting. From UE and eNBperspective, the operations related to periodic CSI reporting isidentical to legacy operation.

The full, large 2D port layout CSI measurements of up to 64 ports oreven more is only present in the aperiodic reports. Since A-CSI iscarried over PUSCH, the payload can be much larger than the small 11-bitlimit of the P-CSI using PUCCH format 2.

CSI-RS Resource Allocation for a 2D Antenna Array

It has been agreed that for 12 or 16 ports, a CSI-RS resource for classA CSI reporting is composed as an aggregation of K CSI-RS configurationseach with N ports. In case of CDM-2, the K CSI-RS resourceconfigurations indicate CSI-RS RE locations according to legacy resourceconfigurations in TS36.211. For 16 ports:(N,K)=(8,2),(2,8)

For 12 port construction:(N,K)=(4,3),(2,6)

The ports of the aggregated resource correspond to the ports ofcomponent resources according to the following:

The aggregated port numbers are 15, 16, . . . 30 (for 16 CSI-RS ports)The aggregated port numbers are 15, 16, . . . 26 (for 12 CSI-RS ports)

CSI-RS Antenna Port Numbering

For a given P antenna ports, the Rel-10,12 and 13 precoding codebooksare designed so that the P/2 first antenna ports (e.g. 15-22) should mapto a set of co-polarized antennas and the P/2 last antenna ports (e.g.16-30) are mapped to another set of co-polarized antennas, with anorthogonal polarization to the first set. This is thus targetingcross-polarized antenna arrays. FIG. 12 illustrates antenna portnumbering for a case of P=8 ports.

Hence, the codebook principles for the rank 1 case are that a DFT “beam”vector is chosen for each set of P/2 ports and a phase shift with QPSKalphabet is used to co-phase the two sets of antenna ports. A rank 1codebook is thus constructed as

$\begin{pmatrix}a \\{ae}^{i\;\omega}\end{pmatrix}\quad$where a is a length P/2 vector that forms a beam for the first andsecond polarizations respectively and ω is a co-phasing scalar thatco-phases the two orthogonal polarizations.Using CSI-RS Signals in NR

In NR, the CSI-RS signal needs to be designed and used for at leastsimilar purposes as in LTE. However, the NR CSI-RS is expected tofulfill additional purposes such as beam management. Beam management isa process whereby eNB and UE beams are tracked which includes finding,maintaining, and switching between suitable beams as UEs move bothwithin and between the coverage areas of multi-beam transmit-receivepoints (TRPs). This is accomplished by UEs performing measurements onthe CSI-RS reference signals and feeding these measurements back to thenetwork for the purposes of beam management decisions.

It is thus a problem how to design a CSI-RS that can be used for “LTEtype” of functionality as well as for beam management functionality withboth digital and analog beamforming.

An additional point of difference between NR and LTE is that NR willsupport flexible numerology, i.e., scalable sub-carrier spacing (SCS)with a nominal value of 15 kHz. The nominal value is scalable in powersof 2, i.e., fSC=15*2n kHz where n=−2, −1, 0, 1, 2, 3, 4, 5. This affectsthe CSI-RS structure, as larger subcarrier spacings mean that resourceelements (REs) can become more spread out in the frequency dimension andthis results in a larger distance in frequency between CSI-RS. It isthus a problem how to design CSI-RS to be able to adjust the frequencydensity depending on the SCS.

One more possible point of difference is that NR may support a shortertransmission duration than LTE. The NR transmission duration is a slotwhere a slot can be either 7 or 14 OFDM symbols long. In contrast, thetransmission duration in LTE is fixed at one subframe which equals 14symbols.

Additionally, because there is no common reference signals (CRS) in NR,the placement of CSI-RS in NR is not restricted to avoid collisions withNR. Thus, greater flexibility may be used in the design of CSI-RS forNR.

SUMMARY

Several of the techniques and apparatus described herein address theabove issues and provide greater flexibility in the design and use ofCSI-RS for NR.

Some embodiments of the presently disclosed invention include a methodof configuring, in a network node of a wireless communication network, areference signal resource used to perform channel-state information(CSI) measurements with one or more wireless devices in the wirelesscommunication network. This method comprises the steps of aggregating areference signal resource in one or more of a frequency and a timedomain, and adjusting a density characteristic of the aggregatedreference signal resource that is to be transmitted to the one or morewireless devices. In some embodiments, the density characteristicincludes at least one of: a number of ports in a radio access node fromwhich the aggregated reference signal resource is to be transmitted; asampling rate or sample interval of the aggregated reference signalresource; and a frequency band for which the aggregated reference signalresource is allocated. This adjustment of the density characteristic, insome embodiments, may be based at least partially on at least one of: asubcarrier spacing control parameter; a beam management controlparameter; and a channel variation measurement parameter.

Other embodiments of the presently disclosed invention include a method,in a network node of a wireless communication network, of selectivelyconfiguring variable density reference signal resources used to transmitreference signals for measurement by a wireless device in the wirelesscommunications network, according to one or more of the techniquesdescribed herein. In some of these embodiments, the method comprisesselecting a resource aggregation from among a plurality of resourceaggregations, where each of the plurality of differing resourceaggregations has a differing number of resource units and comprises afirst number i of OFDM symbols that carry resource units within eachtransmission slot and a second number j of resource units per each ofthe first number of OFDM symbols, per each of one or more resourceblocks. Each resource block comprises a predetermined number ofsubcarriers in the frequency domain. The method further includesselecting a third number p of ports, among which the resource unitswithin each transmission slot are allocated. A reference signal resourceconfiguration having a reference signal port density D per resourceblock is thereby configured. The method further comprises transmitting,for each of the p ports, a reference signal to the wireless device in atleast one transmission slot, using the resource units allocated to therespective port in the at least one transmission slot. In someembodiments, the method may further comprise signaling an indication ofthe reference signal resource configuration to the wireless device.

In some embodiments, the resource units referred to above each consistof two adjacent OFDM resource elements. In some embodiments, the firstnumber i of OFDM symbols within each transmission slot are contiguous.

In some embodiments, transmitting the reference signal for each of the pports comprises applying an orthogonal cover code to a predeterminedsignal sequence before transmitting the reference signal. In someembodiments, the method further comprises selecting a subsampling factorSF from a plurality of subsampling factors, each subsampling factorcorresponding to a different minimum spacing of reference signal symbolsin the frequency domain, thereby defining a reduced density referencesignal configuration having a reduced reference signal port density D′per resource block, where D′=D/SF. In these embodiments, transmittingthe reference signal to the wireless device in at least one transmissionslot comprises transmitting the reference signals according to thereduced density reference signal configuration.

Other embodiments of the present invention include apparatusescorresponding to the above-summarized methods and configured to carryout one or more of these methods, or variants thereof. Thus, embodimentsinclude a network node for use in a wireless communication network, thenetwork node being adapted to configure a reference signal resource usedto perform (CSI) measurements with one or more wireless devices in thewireless communication network by: aggregating a reference signalresource in one or more of a frequency and a time domain; and adjustinga density characteristic of the aggregated reference signal resourcethat is to be transmitted to the one or more wireless devices. In someembodiments, this network node may comprise a processing circuit and amemory operatively coupled to the processing circuit and storing programcode for execution by the processing circuit, whereby the network nodeis configured to carry out these operations.

Other embodiments include another network node, for use in a wirelesscommunication network, this network node being adapted to selectivelyconfigure variable density reference signal resources used to transmitreference signals for measurement by a wireless device in the wirelesscommunications network by: selecting a resource aggregation from among aplurality of resource aggregations, each of the plurality of differingresource aggregations having a differing number of resource units andcomprising a first number i of OFDM symbols that carry resource unitswithin each transmission slot and a second number j of resource unitsper each of the first number of OFDM symbols, per each of one or moreresource blocks, each resource block comprising a predetermined numberof subcarriers in the frequency domain; and selecting a third number pof ports, among which the resource units within each transmission slotare allocated. By performing these selecting operations, a referencesignal resource configuration having a reference signal port density Dper resource block is thereby defined. This network node is furtheradapted to transmit, for each of the p ports, a reference signal to thewireless device in at least one transmission slot, using the resourceunits allocated to the respective port in the at least one transmissionslot. Again, in some embodiments, this network node may comprise aprocessing circuit and a memory operatively coupled to the processingcircuit and storing program code for execution by the processingcircuit, whereby the network node is configured to carry out theseoperations.

Still other embodiments comprise systems that include one or more of thenetwork nodes summarized above, in addition to one or more wirelessdevices. Yet other embodiments comprise computer program products andcomputer-readable media storing computer program products, where thecomputer program products comprise program instructions for execution bya processor of a network node, such that the network node is therebyoperative to carry out one or more of the methods summarized above orvariants thereof, as detailed further, below.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate selected embodiments of the disclosed subjectmatter. In the drawings, like reference labels denote like features.

FIG. 1 is a diagram illustrating an LTE network.

FIG. 2 is a diagram illustrating a wireless communication device.

FIG. 3 is a diagram illustrating a radio access node.

FIG. 4 is a flowchart illustrating a method of operating a network node.

FIG. 5 is a diagram illustrating a network node.

FIG. 6 is a schematic diagram of an example Orthogonal FrequencyDivision Multiplexing (OFDM) downlink physical resource.

FIG. 7 is a schematic diagram of an example OFDM time-domain structure.

FIG. 8 is a schematic diagram of an example OFDM downlink subframe.

FIG. 9 is a functional block diagram of a spatial multiplexingoperation.

FIG. 10 is a graphical illustration of example resource element gridsover an RB pair.

FIG. 11 is a graphical illustration of an example antenna array and itscorresponding port layout.

FIG. 12 is a graphical illustration of an example numbering scheme forantenna ports.

FIG. 13 is an example signaling diagram between a radio access node of awireless communications network and a wireless communication device.

FIG. 14 is another example signaling diagram between a radio access nodeof a wireless communications network and a wireless communicationdevice.

FIG. 15 is a graphical illustration of an OFDM symbol having six CSI-RSunits in one PRB.

FIG. 16 is a graphical illustration of two different NR slot sizes andthe example location of CSI-RS units therein.

FIG. 17 is a graphical illustration of various resource allocationconfigurations in which CSI-RS units may be aggregated.

FIG. 18 is a graphical illustration of various example port numbermappings that correspond to the resource allocation configurations ofFIG. 17.

FIG. 19 is a graphical illustration of two possible comb patterns orstructures resulting from a subsampling of an aggregated CSI-RSresource.

FIG. 20 is a graphical illustration of another possible comb pattern orstructure resulting from a subsampling of an aggregated CSI-RS resource.

DETAILED DESCRIPTION

The following description presents various embodiments of the disclosedsubject matter. These embodiments are presented as teaching examples andare not to be construed as limiting the scope of the disclosed subjectmatter. For example, certain details of the described embodiments may bemodified, omitted, or expanded upon without departing from the scope ofthe described subject matter.

Radio Node: As used herein, a “radio node” is either a radio access nodeor a wireless device.

Controlling Node: As used herein, a “controlling node” either a radioaccess node or a wireless device used to manage, control or configureanother node.

Radio Access Node: As used herein, a “radio access node” is any node ina radio access network of a cellular communications network thatoperates to wirelessly transmit and/or receive signals. Some examples ofa radio access node include, but are not limited to, a base station(e.g., an enhanced or evolved Node B (eNB) in a Third GenerationPartnership Project (3GPP) Long Term Evolution (LTE) network), ahigh-power or macro base station, a low-power base station (e.g., amicro base station, a pico base station, a home eNB, or the like), and arelay node.

Core Network Node: As used herein, a “core network node” is any type ofnode in a Core Network (CN). Some examples of a core network nodeinclude, e.g., a Mobility Management Entity (MME), an Evolved-ServingMobile Location Center (E-SMLC), a Packet Data Network (PDN) Gateway(P-GW), a Service Capability Exposure Function (SCEF), or the like.

Wireless Device: As used herein, a “wireless device” is any type ofdevice that is capable of wirelessly transmitting and/or receivingsignals to/from another wireless device or to/from a network node in acellular communications network to obtain has access to (i.e., be servedby) the cellular communications network. Some examples of a wirelessdevice include, but are not limited to, a User Equipment (UE) in a 3GPPnetwork, a Machine Type Communication (MTC) device, an NB-IoT device, aFeMTC device, etc.

Network Node: As used herein, a “network node” is any node that iseither part of the radio access network or the CN of a cellularcommunications network/system or a test equipment node.

Signaling: As used herein, “signaling” comprises any of: high-layersignaling (e.g., via Radio Resource Control (RRC) or a like),lower-layer signaling (e.g., via a physical control channel or abroadcast channel), or a combination thereof. The signaling may beimplicit or explicit. The signaling may further be unicast, multicast orbroadcast. The signaling may also be directly to another node or via athird node.

The differences between LTE and NR drive a design for CSI-RS that isvery flexible in terms of the CSI-RS resource density both in the timeand frequency dimensions. For example, for large subcarrier spacings(e.g., 240 kHz), it is necessary to have a significantly higher densityin the frequency domain than for the nominal subcarrier spacing of 15kHz so as to maintain similarly spaced samples of the frequencyselective channel. On the other hand, for beam management purposes, itis often necessary to have a fairly spare density in frequency. Hence,what is needed for NR is a very flexible and configurable/controllabledensity to suit a wide range of use cases. This high flexibility islacking from the LTE CSI-RS design.

A CSI-RS design with a highly flexible/controllable CSI-RS antenna portdensity is desirable for NR. According to some of the presentlydisclosed techniques, the density may be controlled in one or both oftwo general ways:

-   -   1) The number of ports assigned to an aggregated CSI-RS resource        is configurable by the network. Fewer ports assigned to a        resource translates to higher port density and vice versa.    -   2) Subsampling of the aggregated CSI-RS in the frequency domain        is configurable by network. Increased subsampling of a resource        translates to lower port density and vice versa.

Flexible/controllable CSI-RS port density allows a single CSI-RSframework to be easily adapted to suit a wide range of use cases anddeployment scenarios necessary for NR. The foregoing two general controlfeatures may be used individually or jointly to suit the scenario ofinterest. Such flexibility improves NR system performance across allsub-carrier spacings and operating carrier frequencies, for both analogbeamforming and digital front ends.

According to some embodiments of the presently disclosed techniques, abasic CSI-RS “unit” may be defined as two adjacent resource elements(REs) contained within one OFDM symbol in a slot. This is a modularapproach, which then can be extended to support various needs and usecases of a NR deployment. A technical advantage of the basic unit beingtwo REs adjacent in frequency but in same symbol, compared to thedifferent approach used in LTE, is better flexibility in overlappingthese with other reference signals, such as the new tracking referencesignal designed for NR.

The CSI-RS units may be aggregated to form a CSI-RS resource. The CSI-RSresource is signaled from the network (gNB, eNB, TRP, . . . ) to the UEand the UE then performs CSI measurements on this CSI-RS resource andthe UE feeds back CSI measurement reports to the network. The networkthen uses this information for link adaptation and/or beam selectionand/or beam management.

FIG. 13 depicts a signaling diagram between a radio access node of awireless communications network (denoted “Network/gNB”) and a wirelesscommunication device (denoted “Terminal/UE”) in which the networkconfigures CSI-RS resources for CSI feedback and transmits CSI-RS to thewireless communication device/UE. Measurements are then performed in theUE, and a CSI report is sent as feedback to the network. Data may thenbe transmitted from the radio access node to the wireless communicationdevice, e.g., based on a precoder that is determined from the CSIreports.

FIG. 14 depicts a similar signaling diagram. However, in FIG. 14, a beammanagement setup is also depicted, in which the wireless communicationdevice selects beams. More particularly, the CSI-RS resource contains Nports which are divided into B beams, so that each beam has N/B ports.The wireless communication device selects the desired subset of N/Bports, i.e. the beam, to use for the CSI feedback.

FIG. 15 depicts an OFDM symbol in a slot having six CSI-RS units thatfit within one PRB (12 subcarriers). Each different color represents adifferent unit. A length-6 bitmap may be used to indicate from thenetwork to the UE whether each of the units or combinations(aggregations) of units are part of a CSI-RS resource or not. The bitmapvalues for each individual CSI-RS unit are shown in Table 1 below.

TABLE 1 Bitmap values for each individual CSI-RS unit CSI-RS Length-6Unit Bitmap 0 100000 1 010000 2 001000 3 000100 4 000010 5 000001

The location of the CSI-RS units within a slot are described inspecifications by the “anchor locations” listed in

Table 2 below. In each row of this table, the first value of the anchorlocation indicates a subcarrier index and the second value ‘x’ indicatesan OFDM symbol index where x={0, 1, 2, . . . , 6} in the case of a7-symbol slot and x={0, 1, 2, . . . , 13} for the case of a 14 symbolslot. Example locations for the two different NR slot sizes are shown inFIG. 16.

TABLE 2 Anchor locations for CSI-RS units. CSI-RS Anchor Unit Location 0(11, x)  1 (9, x) 2 (7, x) 3 (5, x) 4 (3, x) 5 (1, x)

A CSI-RS resource is defined as an aggregation of CSI-RS units andfurther also with a port assignment which is also signaled from thenetwork to the UE. Moreover, a CSI-RS resource may also include theresource blocks for which the CSI-RS resource is valid. In some cases,the CSI-RS does not span the whole system bandwidth but only a partialbandwidth. Note that the figures shown in the present application onlyshow a single or two RBs, but these RB patterns may be repeated over thewhole configured set of RBs (typically the whole system bandwidth, orthe bandwidth for which the UE supports CSI measurements).

In the next two subsections, the flexible aggregation part is describedfollowed by the flexible port assignment part. Together these compriseone aspect of several embodiments of the presently disclosed techniquesand apparatus. Another aspect of some embodiments (flexible resourcesubsampling) is described in the 3rd sub-section.

Flexible Resource Aggregation

A CSI-RS resource in several embodiments of the present invention isdefined as the flexible aggregation of (a) resource units per OFDMsymbol, and (b) OFDM symbols plus a port assignment to the aggregatedresource. The definition of the CSI-RS may possibly also include thesupported set of multiple RBs over where this CSI-RS port extends.

For (b), the aggregated OFDM symbols may be either contiguous/adjacentor non-contiguous. For ease of discussion, it is assumed that the OFDMsymbols comprising the resource are contained within the same slot.However, in some embodiments they may span more than one slot. A usecase for non-contiguous OFDM symbols in a CSI-RS resource within a slotcan be to support frequency error estimation and tracking for the UE(which requires some time spacing between the reference signals foraccuracy).

FIG. 17 shows example aggregations for the case of 1, 2, and 4contiguous OFDM symbols. The bitmap at the top of each box indicates theCSI-RS units that form the basis of the aggregation per OFDM symbol. Forexample, bitmap 110011 indicates that the aggregation is formed from 4different CSI-RS units: 1 (the top two subcarriers in each OFDM symbol),2 (the next two subcarriers), 5 (the pair of subcarriers just above thebottom two subcarriers), and 6 (the bottom two subcarriers).

With such resource aggregations that span both time (OFDM symbols) andfrequency (subcarriers, i.e. units), in some embodiments, orthogonalcover codes (OCCs) may be applied as in LTE within and/or between CSI-RSunits. The use of OCCs is useful in order to collect more energy perport if they are applied across time. If they are applied acrossfrequency, larger CSI-RS power boosting may be applied without violatinga potential fixed threshold on the peak to average power ratio acrossresource elements.

Flexible Port Assignment

In order to control the port density in an aggregated CSI-RS resource, aflexible port assignment scheme is adopted in some embodiments of thepresently disclosed techniques. With this approach, a network node canassign a variable number of ports to an aggregated resource within aCSI-RS resource.

If a small number of ports is assigned to a larger aggregated resource,then a high port density is achieved, since each port is represented ina large number of resource elements. This is useful in the case of largesub-carrier spacing. Hence, it is possible to control the port density D(defined as number of resource elements per port per resource block)depending on the use case with this configuration.

Several examples are shown in each box in FIG. 17. For example, in the3rd box from the left on the bottom row, the assignment of 4 ports, 8ports, and 16 ports is shown. In each of these aggregations, there are16 REs, hence the port density, D, in the three cases is 4, 2, and 1REs/port/PRB, respectively. In all cases when the number of ports isless than the number of REs, the port density is greater than 1RE/port/PRB. This is beneficial for larger subcarrier spacings so as tomaintain similarly spaced samples of the channel in the frequency domaincompared to the case if a smaller subcarrier spacing was used.

FIG. 18 shows example port number mappings for several of the resourceallocations shown in FIG. 17. In one embodiment, the port numbers aremapped across frequency first (CSI-RS units) and then across time (OFDMsymbols). As can be seen, a given port number appears D times within theresource which is consistent with the definition of port density interms of REs/port/PRB.

Flexible Resource Subsampling

In the previous two subsections entitled “Flexible Resource Aggregation”and “Flexible Port Assignment,” methods for achieving flexible andcontrollable density D of greater than or equal to 1 RE/port/PRB isdescribed. In this subsection, a second aspect of certain embodiments isdescribed whereby flexible density reduction capable of producingdensities of less than 1 RE/port/PRB is described (D<1). This is usefulfor several purposes. One is for beam management purposes, where often abeam sweep is used to discover the “direction” of the UE for use inbeamforming future control and data transmissions. For this type ofapplication, it is useful to have a relatively sparse CSI-RS density inthe frequency dimension. A reason is that often analog beamforming isused (at high carrier frequencies such as 28 GHz), and the beam is thuswideband and the corresponding RE used for an CSI-RS antenna port can bespread out over the bandwidth (this may be referred to, in relativeterms, as a low frequency sampling rate or, equivalently, a largesampling interval).

Another application for spare CSI-RS density is in scenarios where thechannel varies relatively slowly in the frequency dimension, hencefrequent sampling in frequency is not necessary. A sparser pattern canlead to higher data transmission peak rates since more resources areavailable for multiplexing data symbols with the CSI-RS symbols.

Flexible and controllable density reduction also for D<1 is achieved incertain embodiments of the invention by subsampling the aggregatedCSI-RS resource by a subsampling factor SF=1, 2, 3, 4, . . . where SF=1means no subsampling and SF>1 means that a CSI-RS symbol is located atmost every SF subcarriers in the frequency domain. Subsampling resultsin a frequency “comb” structure where the spacing of the comb tines isequal to SF. It will be appreciated that a higher SF, i.e., a highersubsampling factor, results in a higher sampling rate, in that theCSI-RS symbols are more closely spaced, i.e., having a smaller sampleinterval.

FIG. 19 shows an example comb for a 16 RE resource using SF=2 (twodifferent comb offsets that are possible for SF=2 are shown). If 16ports are assigned to this aggregated resource, then the use of SF=2results in a density of D=½ which is less than 1 RE/port/PRB as desired.

When such a comb structure is used, there are SF-1 possibilities forintroducing an offset of the comb. In FIG. 19 the two possible combpatterns are shown, one with no offset and one with offset value O=1.Use of a comb offset can be beneficial in order to allocate orthogonalcombs to two different users—another motivation for density reduction.

Note that in FIG. 19, the value m is a PRB index where m spans aparticular bandwidth. This may be the whole system bandwidth or aportion thereof, for example a partial band allocated to a given user.In this example, the CSI-RS units span two different PRBs, sincesubsampling with SF=2 is used. Generally, the number of PRBs spanned bythe CSI-RS units is equal to SF.

Yet another example of resource subsampling is shown FIG. 20 wheresubsampling factor SF=4 is used on a pattern using all 6 CSI-RS units(bitmap=111111) and 2 ports are assigned. With zero samples in betweenthe “stripes” in this figure, the pattern is referred to as interleavedfrequency division multiple access (IFDMA). This type of pattern isuseful for beam sweeping operations performed in the context of beammanagement. Here, a different eNB transmit (Tx) beam can be used in eachOFDM symbol. Then within each OFDM symbol, the UE can sweep its Rx beam4 times (equal to the SF) since the IFDMA pattern creates a periodictime domain waveform with period=4 within each OFDM symbol.

Using the above techniques allows for a very flexible and scalabledefinition of an CSI-RS resource for NR which can support a wide rangeof carrier frequencies (1-100 GHz), implementation choices (digital oranalog beamforming). For example, embodiments of the presently disclosedtechniques allow for definition of the CSI-RS resource according to oneor more of the following aspects:

-   -   1. Aggregated resource units in frequency domain (one OFDM        symbol)        -   a. Described by a length-6 bitmap indicating a particular            combination of unit 1, 2, 3, 4, 5, and 6    -   2. Aggregated resource units in time domain        -   a. OFDM symbol indices over which to aggregate    -   3. Number of ports assigned to the aggregated resource    -   4. Subsample factor SF=1, 2, 3, 4, . . . and Comb Offset=0, 1, .        . . , SF-1    -   5. A frequency band for which the CSI-RS resource is allocated        (partial band, whole band)    -   6. OCC configuration (if used)

The described embodiments may be implemented in any appropriate type ofcommunication system supporting any suitable communication standards andusing any suitable components. As one example, certain embodiments maybe implemented in an LTE network, such as that illustrated in FIG. 1.

Referring to FIG. 1, a communication network 100 comprises a pluralityof wireless communication devices 105 (e.g., conventional UEs, machinetype communication [MTC]/machine-to-machine [M2M] UEs) and a pluralityof radio access nodes 110 (e.g., eNodeBs or other base stations).Communication network 100 is organized into cells 115, which areconnected to a core network 120 via corresponding radio access nodes110. Radio access nodes 110 are capable of communicating with wirelesscommunication devices 105 along with any additional elements suitable tosupport communication between wireless communication devices or betweena wireless communication device and another communication device (suchas a landline telephone).

Although wireless communication devices 105 may represent communicationdevices that include any suitable combination of hardware and/orsoftware, these wireless communication devices may, in certainembodiments, represent devices such as an example wireless communicationdevice illustrated in greater detail by FIG. 2. Similarly, although theillustrated radio access node may represent network nodes that includeany suitable combination of hardware and/or software, these nodes may,in particular embodiments, represent devices such as the example radioaccess node illustrated in greater detail by FIG. 3.

Referring to FIG. 2, a wireless communication device 200 comprises aprocessor 205, a memory, a transceiver 215, and an antenna 220. Incertain embodiments, some or all of the functionality described as beingprovided by UEs, MTC or M2M devices, and/or any other types of wirelesscommunication devices may be provided by the device processor executinginstructions stored on a computer-readable medium, such as the memoryshown in FIG. 2. Alternative embodiments may include additionalcomponents beyond those shown in FIG. 2 that may be responsible forproviding certain aspects of the device's functionality, including anyof the functionality described herein. It will be appreciated that thedevice processor 205 may comprise one or more microprocessors,microcontrollers, digital signal processors, and the like, with theseone or more processing elements being configured to execute program codestored in memory 210, to control the transceiver 215 and to execute allor some of the functionality described herein, and may include, in someembodiments, hard-coded digital logic that carries out all or some ofthe functionality described herein. The term “processing circuit” isused herein to refer to any one of these combinations of processingelements.

Referring to FIG. 3, a radio access node 300 comprises a node processor305, a memory 310, a network interface 315, a transceiver 320, and anantenna 325. Again, it will be appreciated that the node processor 305may comprise one or more microprocessors, microcontrollers, digitalsignal processors, and the like, with these one or more processingelements being configured to execute program code stored in memory 310,to control the transceiver 320 and the network 315 and to execute all orsome of the functionality described herein, and may include, in someembodiments, hard-coded digital logic that carries out all or some ofthe functionality described herein. This functionality includes, forexample, the operations shown in the flowcharts of FIGS. 4 and 5. Theterm “processing circuit” is used herein to refer to any one of thesecombinations of processing elements.

Thus, in certain embodiments, some or all of the functionality describedas being provided by a base station, a node B, an eNodeB, and/or anyother type of network node may be provided by node processor 305executing instructions stored on a computer-readable medium, such asmemory 310 shown in FIG. 3. Again, this functionality includes, forexample, the operations shown in the flowcharts of FIGS. 4 and 5.Alternative embodiments of radio access node 300 may comprise additionalcomponents to provide additional functionality, such as thefunctionality described herein and/or related supporting functionality.

FIG. 4 is a flowchart illustrating an example method 400 of operating anetwork node (e.g., a radio access node 110). The method 400 comprises astep 405 in which a reference signal resource is aggregated in one ormore of a frequency and a time domain. The method further comprises astep 410 in which a density characteristic of the aggregated referencesignal resource that is to be transmitted to the one or more wirelessdevices (105) is adjusted. The method further comprises a step 415 inwhich a reference signal is transmitted to each of the one or morewireless devices (105), using the aggregated reference signal resourcewith the adjusted density characteristic. The method may still furthercomprise, in some embodiments, signaling an indication of the aggregatedreference signal resource with the density characteristic to the one ormore wireless devices (105).

FIG. 5 illustrates another flowchart, this flowchart showing an examplemethod 500, in a network node (110) of a wireless communication network,of selectively configuring variable density reference signal resourcesused to transmit reference signals for measurement by a wireless devicein the wireless communications network, according to one or more of thetechniques described herein.

As seen at block 510, the illustrated method comprises selecting aresource aggregation from among a plurality of resource aggregations,where each of the plurality of differing resource aggregations has adiffering number of resource units and comprises a first number i ofOFDM symbols that carry resource units within each transmission slot anda second number j of resource units per each of the first number of OFDMsymbols, per each of one or more resource blocks. Each resource blockcomprises a predetermined number of subcarriers in the frequency domain.

As seen at block 520, the method further comprises selecting a thirdnumber p of ports, among which the resource units within eachtransmission slot are allocated. With the performing of the steps shownin blocks 510 and 520, as described above, a reference signal resourceconfiguration having a reference signal port density D per resourceblock is thereby configured.

As seen at block 540, the method further comprises transmitting, foreach of the p ports, a reference signal to the wireless device in atleast one transmission slot, using the resource units allocated to therespective port in the at least one transmission slot. In someembodiments, the method may further comprise signaling an indication ofthe reference signal resource configuration to the wireless device, asshown at block 530.

In some embodiments, the resource units referred to above each consistof two adjacent OFDM resource elements. In some embodiments, the firstnumber i of OFDM symbols within each transmission slot are contiguous.

In some embodiments, transmitting the reference signal for each of the pports comprises applying an orthogonal cover code to a predeterminedsignal sequence before transmitting the reference signal. In someembodiments, the method further comprises selecting a subsampling factorSF from a plurality of subsampling factors, each subsampling factorcorresponding to a different minimum spacing of reference signal symbolsin the frequency domain, thereby defining a reduced density referencesignal configuration having a reduced reference signal port density D′per resource block, where D′=D/SF. In these embodiments, transmittingthe reference signal to the wireless device in at least one transmissionslot comprises transmitting the reference signals according to thereduced density reference signal configuration.

As described above, the exemplary embodiments provide both methods andcorresponding apparatuses consisting of various modules providingfunctionality for performing the steps of the methods. The modules maybe implemented as hardware (embodied in one or more chips including anintegrated circuit such as an application specific integrated circuit),or may be implemented as software or firmware for execution by aprocessor. In particular, in the case of firmware or software, theexemplary embodiments can be provided as a computer program productincluding a computer-readable storage medium embodying computer programcode (i.e., software or firmware) thereon for execution by the computerprocessor. The computer readable storage medium may be non-transitory(e.g., magnetic disks; optical disks; read only memory; flash memorydevices; phase-change memory) or transitory (e.g., electrical, optical,acoustical or other forms of propagated signals-such as carrier waves,infrared signals, digital signals, etc.). The coupling of a processorand other components is typically through one or more busses or bridges(also termed bus controllers). The storage device and signals carryingdigital traffic respectively represent one or more non-transitory ortransitory computer readable storage medium. Thus, the storage device ofa given electronic device typically stores code and/or data forexecution on the set of one or more processors of that electronic devicesuch as a controller.

Although the embodiments and its advantages have been described indetail, it should be understood that various changes, substitutions, andalterations can be made herein without departing from the spirit andscope thereof as defined by the appended claims. For example, many ofthe features and functions discussed above can be implemented insoftware, hardware, or firmware, or a combination thereof. Also, many ofthe features, functions, and steps of operating the same may bereordered, omitted, added, etc., and still fall within the broad scopeof the various embodiments.

LIST OF ABBREVIATIONS

TRP—Transmission/Reception Point

UE—User Equipment

NW—Network

BPL—Beam pair link

BLF—Beam pair link failure

BLM—Beam pair link monitoring

BPS—Beam pair link switch

RLM—radio link monitoring

RLF—radio link failure

PDCCH—Physical Downlink Control Channel

RRC—Radio Resource Control

CRS—Cell-specific Reference Signal

CSI-RS—Channel State Information Reference Signal

RSRP—Reference signal received power

RSRQ—Reference signal received quality

gNB—NR base station

PRB—Physical Resource Block

RE—Resource Element

What is claimed is:
 1. A method of configuring, in a network node of awireless communication network, a reference signal resource used toperform channel-state information, CSI, measurements by one or morewireless devices in the wireless communication network, the methodcomprising: aggregating a CSI-RS resource in a frequency domain, whereinthe aggregated CSI-RS resource has an associated port densitycharacteristic that defines a number of resource elements per port perresource block; and signaling an indication of the aggregated CSI-RSresource and an indication of the associated port density characteristicto the one or more wireless devices, wherein the indication of theaggregated CSI-RS resource includes a bitmap.
 2. The method of claim 1,wherein the port density characteristic is determined based at leastpartially on a beam management control parameter value.
 3. The method ofclaim 1, further comprising transmitting a CSI-RS to at least one of thewireless devices, using the aggregated CSI-RS resource with theassociated port density characteristic.
 4. The method of claim 1,wherein the CSI-RS resource is further aggregated in a time domain. 5.The method of claim 1, wherein each bit in the bitmap indicates whethera component located at a corresponding subcarrier index is part of theaggregated CSI-RS resource.
 6. A network node for use in a wirelesscommunication network, the network node comprising: a processor; and amemory containing instructions that, when executed by the processor,cause the network node to configure a reference signal resource used toperform channel-state information, CSI, measurements with one or morewireless devices in the wireless communication network by: aggregating aCSI-RS resource in a frequency domain, wherein the aggregated CSI-RSresource has an associated port density characteristic that defines anumber of resource elements per port per resource block; and signalingan indication of the aggregated CSI-RS resource and an indication of theassociated port density characteristic to the one or more wirelessdevices, wherein the indication of the aggregated CSI-RS resourceincludes a bitmap.
 7. The network node of claim 6, wherein the portdensity characteristic is determined based at least partially on a beammanagement control parameter value.
 8. The network node of claim 6,wherein the instructions, when executed by the processor, further causethe network node to transmit a CSI-RS to each of the wireless devices,using the aggregated CSI-RS resource with the associated port densitycharacteristic.
 9. The network node of claim 6, wherein the CSI-RSresource is further aggregated in a time domain.
 10. The network node ofclaim 6, wherein each bit in the bitmap indicates whether a componentlocated at a corresponding subcarrier index is part of the aggregatedCSI-RS resource.
 11. A method of a wireless device receiving and usingreference signals in a wireless communication network, the methodcomprising: receiving a first indication of: a first aggregated CSI-RSresource, and a first density characteristic associated with the firstaggregated CSI-RS resource and that defines a number of resourceelements per port per resource block, the first indication including afirst bitmap that indicates how a plurality of resource units areaggregated in at least a frequency domain to form the first aggregatedCSI-RS resource; receiving a first CSI-RS based at least partially onthe first indication of the first aggregated CSI-RS resource and thefirst density characteristic; receiving a second indication of: a secondaggregated CSI-RS resource, and a second density characteristicassociated with the second aggregated CSI-RS resource and that defines anumber of resource elements per port per resource block, the secondindication including a second bitmap that indicates how a plurality ofresource units are aggregated in at least a frequency domain to form thesecond aggregated CSI-RS resource; receiving a second CSI-RS based atleast partially on the second indication of the second aggregated CSI-RSresource and the second density characteristic.
 12. The method of claim11, wherein the wireless device uses the first CSI-RS to estimatechannel state information and uses the second CSI-RS for beam managementreporting.
 13. The method of claim 11, wherein the second densitycharacteristic is based at least partially on a beam management controlparameter value.
 14. The method of claim 11, wherein each bit in thefirst bitmap indicates whether a component located at a correspondingsubcarrier index is part of the first aggregated CSI-RS resource, andwherein each bit in the second bitmap indicates whether a componentlocated at a corresponding subcarrier index is part of the secondaggregated CSI-RS resource.
 15. A wireless device for use in a wirelesscommunication network, the wireless device comprising: a processor; anda memory containing instructions that, when executed by the processor,cause the wireless device to estimate channel state information andperform beam management reporting by: receiving a first indication of: afirst aggregated CSI-RS resource, and a first density characteristicassociated with the first aggregated CSI-RS resource, the firstindication including a first bitmap that indicates how a plurality ofresource units are aggregated in at least a frequency domain to form thefirst aggregated CSI-RS resource; receiving a first CSI-RS based atleast partially on the first indication of the first aggregated CSI-RSresource and the first density characteristic; receiving a secondindication of: a second aggregated CSI-RS resource, and a second densitycharacteristic associated with the second aggregated CSI-RS resource andthat defines a number of resource elements per port per resource block,the second indication including a second bitmap that indicates how aplurality of resource units are aggregated in at least a frequencydomain to form the second aggregated CSI-RS resource; receiving a secondCSI-RS based at least partially on the second indication of the secondaggregated CSI-RS resource and the second density characteristic. 16.The wireless device of claim 15, wherein the instructions, when executedby the processor, further cause the wireless device to use the firstCSI-RS to estimate channel state information and use the second CSI-RSfor beam management reporting.
 17. The wireless device of claim 15,wherein the second density characteristic is based at least partially ona beam management control parameter value.
 18. The wireless device ofclaim 15, wherein each bit in the first bitmap indicates whether acomponent located at a corresponding subcarrier index is part of thefirst aggregated CSI-RS resource, and wherein each bit in the secondbitmap indicates whether a component located at a correspondingsubcarrier index is part of the second aggregated CSI-RS resource.